Generally, luster pigments are known in the art for use in various applications such as automotive finishes, coatings and other pigment applications.
Such luster pigments may be formed by deposition of titanium oxide over highly reflective platelet-like aluminum flakes in a water system. The Al flake may include a protection layer of SiO2. TiO2 may be deposited to the Al flake under highly acidic conditions such as at a pH of less than 2.0 such that a hydrolysis reaction for TiO2 may be achieved. However, such a process results in unsatisfactory coatings due to the diffusion of aqueous solution through the SiO2 layer.
Problems associated with the above deposition include both etching of the Al core and a change in pH near the SiO2—Al surface which are undesired for TiO2 deposition. At such low pH, the protons in the aqueous solution can still diffuse through the SiO2 layer and react with the Al core during the typical long deposition period. This side reaction between the proton and Al as well as the resultant pH increase at the core surface renders the deposition of TiO2 difficult. Diffusion of protons through the SiO2 layer may cause a reaction with Al such that hydrogen gas is released causing weak adhesion of the TiO2 particles and the formation of channels or cracks in the SiO2 and TiO2 layers. Additionally, an increase in the pH may cause rapid deposition of the TiO2 layer and formation of large particles of TiO2 which would adversely affect the pigment's properties.
Further increasing the thickness of the protection layer of SiO2 is also undesired as the low refractive index SiO2 would reduce the overall color performance of pigments. There is therefore a need in the art for an improved process and pigment that solves the problems identified above and produces a pigment that has a crack free and uniform TiO2 layer.